In recent years, electronic products have lighter weights, smaller sizes and higher densities. This has increased the demands for various printed boards. Among these printed boards, the demand for a flexible laminate (also referred to as a flexible printed circuit board (FPC) or the like) is particularly increasing. The flexible laminate has such a structure that a circuit constituted by a metal layer is provided on an insulating film such as a polyimide film.
Recent development of mobile phones and the like that are highly functioned and reduced in size has caused an increase in demand for an FPC having high flexibility and dimensional stability more than before. These properties are largely dependent on properties of a polyimide film that serves as a base of the FPC. One example of a conventional polyimide film is a polyimide film made from 4,4′-diaminodiphenylether (hereinafter also referred to as 4,4′-ODA) and pyromellitic dianhydride (hereinafter also referred to as PMDA). This conventional film has an average modulus of elasticity (hereinafter also referred to as modulus of elasticity) of about less than 4 GPa, and has a low stiffness value (i.e., has high flexibility), which stiffness value indicates film strength. However, an average coefficient of linear expansion (hereinafter also referred as coefficient of linear expansion) of the film exceeds 30 ppm, thereby giving the film insufficient dimensional stability (for example, see Patent Literature 1).
A technique to deal with this problem has been disclosed in which copolymerization is performed with a p-phenylenediamine for improving dimensional stability, thereby attaining a polyimide film that has a coefficient of linear expansion close to that of copper which is a metal mainly used (for example, see Patent Literature 2). However, if the coefficient of linear expansion is made close to that of copper, the modulus of elasticity increases in conflict, thereby causing the stiffness value to increase. This causes a problem of losing film flexibility.
As described above, there is a general tendency with the polyimide film that if the modulus of elasticity is low, the stiffness decreases and the coefficient of linear expansion increases. This causes the polyimide film to have poor dimensional stability. For such a reason, it is difficult to strike a balance between flexibility and dimensional stability.
On the other hand, as a typical method for reducing the stiffness value, a method which reduces thickness of the film is known. However, the method which reduces the film thickness has problems such as (i) the film can easily tear during its manufacture, thereby making it difficult to stably produce the film, and (ii) the insulating reliability of the film tends to decrease. Thus, the film thickness that is usable in the method is limited (e.g., see Patent Literature 3).
Therefore, the commercial market is longing for a polyimide film that has no limits in its film thickness, has high flexibility i.e. low stiffness value, and that has high dimensional stability.
Patent Literature 1    Japanese Patent Application Publication, Tokukousho, No. 36-10999 A
Patent Literature 2    Japanese Patent Application Publication, Tokukaihei, No. 1-16832 A
Patent Literature 3    Japanese Patent Application Publication, Tokukaihei, No. 8-156176 A